Question: Omar is 3 times as old as William and is also 18 years older than William. How old is William?
Answer: We can use the given information to write down two equations that describe the ages of Omar and William. Let Omar's current age be $o$ and William's current age be $w$ $o = 3w$ $o = w + 18$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $w$ , and both of our equations have $o$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $3w$ $-$ $ (w + 18)$ which combines the information about $w$ from both of our original equations. Solving for $w$ , we get: $2 w = 18$ $w = 9$.